1. Field of the Invention
The present invention relates to an optical recording and/or reproduction apparatus which optically performs recording and reproduction of information on an optical recording medium, such as an optical disc, optical card, etc.
2. Description of the Related Art
In recent years, as our society has become more information-intensive, an external storage device having a larger capacity has been in increasing demand. In an optical recording of information, a recording density cannot be infinitely increased by reducing the size of a recording pit because of a diffraction limit which is defined by the wavelength of light and the numerical aperture of an objective lens. In order to address such a problem, an optical recording medium having a three-dimensional structure where information is recorded not only on one two-dimensional recording plane but on multiple recording layers superimposed along a direction perpendicular to the two-dimensional plane (optical axis direction), and an optical recording and/or reproduction apparatus have been proposed (Japanese Laid-Open Publication No. 6-28672). FIG. 12 shows an example of a conventional optical recording and/or reproduction apparatus for recording information on/reproducing information from an optical recording medium having a three-dimensional structure.
The optical recording and/or reproduction apparatus 1200 shown in FIG. 12 includes: a light source 101; a beam splitter 360; an objective lens 103; a three-axes stage 761 on which a photosensitive material 760A for recording information is mounted; and a detection section 130. In the photosensitive material 760A, information is three-dimensionally recorded.
As shown in FIG. 12, a laser beam emitted by the light source 101, which is a semiconductor laser, is converged on the photosensitive material 760A by the objective lens 103. In this example, a LiNbO3 crystal, which is a photorefractive crystal, is used as the photosensitive material 760A. The refractive index of the LiNbO3 crystal is changed in relation to a differential value of the intensity distribution of light. The light intensity on the optical axis is proportional to the square root of D (D1/2), where D denotes the distance from the focal point. Accordingly, a variation of the refractive index in the crystal, which occurs when the laser beam is converged on the photosensitive material 760A, is proportional to the cube root of D (D1/3), where D denotes the distance from the focal point. As a result, a variation of the refractive index in the crystal occurs only in the vicinity of a point where the laser beam is converged. With such a variation in the refractive index, information can be three-dimensionally recorded in the photosensitive material (crystal) 760A.
In reproduction of information recorded three-dimensionally in the photosensitive material 760A, a laser beam emitted by the light source 101 is converged at a portion where the refractive index has been varied, and light reflected from that portion is detected by the detection section 130, whereby the three-dimensionally recorded information is reproduced. The three-axes stage 761 is used to move the entire photosensitive material 760A such that the laser beam is converged at a portion in the entire photosensitive material 760A where a desired information is to be recorded.
However, in the above technique, since the three-axes stage 761 is used for positional control in order to achieve three-dimensional recording of information in the photosensitive material 760A, the following problems occur. Firstly, the information recording density in the photosensitive material 760A is limited due to the mechanical precision of the three-axes stage 761. Secondly, moving the entire photosensitive material 760A by using the three-axes stage 761 cannot be achieved speedily, and accordingly, recording/reproduction times become long. Thirdly, when the photosensitive material 760A is replaced with another, the relative positions of the photosensitive material 760A and the three-axes stage 761 are shifted, so that three-dimensionally recorded in formation cannot be read out.
In an optical recording and/or reproduction apparatus for recording information on/reproducing information from an optical recording medium, such as an optical disc or the like, the recording or reproduction time can be shortened by rotating the optical recording medium at a high speed, for example. In order to control a recording position, the position of a guiding groove, which is formed in the optical recording medium at the time of production thereof, is detected by using a push-pull method, a three-beam method, or the like. An objective lens is driven according to the position of the detected guiding groove, whereby the recording position is controlled. The recording position on the optical recording medium and the position of the guiding groove formed on the optical recording medium do not shift even when the recording medium is replaced with another. Thus, replacement of recording media is possible.
However, in such a conventional method for controlling a recording position employed for an optical recording medium, information cannot be recorded in a portion where no guiding groove is formed, although information can be recorded in a portion where a guiding groove is formed.
According to one aspect of the present invention, there is provided an optical recording and/or reproduction apparatus for use with an optical recording medium, the optical recording medium including a recording layer and a substrate which has a guiding portion where concave portions and convex portions are periodically formed along a predetermined direction, the optical recording and/or reproduction apparatus comprising: a light source for emitting a first light beam; an optical system for diffracting a portion of the first light beam so as to output a first diffracted light beam and a second diffracted light beam, and for converging the other portion of the first light beam, thereby outputting a second light beam so as to be converged at a predetermined convergence point; and a controlling section for controlling the optical system, wherein the first and second diffracted light beams interfere with each other to generate interference fringes which extend in a direction substantially perpendicular to the recording layer, the interference fringes include bright portions and dark portions which occur alternately along the predetermined direction, the optical system outputs the first and second diffracted light beams and the second light beam such that the bright portions of the interference fringes form light spots on the guiding portion when the predetermined convergence point of the second light beam is present within the recording layer, the optical recording and/or reproduction apparatus further includes a detection section for detecting the interference fringes reflected by the guiding portion so as to output a detection signal, and the control section controls the optical system based on the detection signal such that the optical spots follow at least either of the concave portions and the convex portions.
In one embodiment of the present invention, the recording layer includes a plurality of recording planes; and the control section controls the optical system such that the predetermined convergence point of the second light beam is positioned on one of the plurality of recording planes.
In another embodiment of the present invention, the frequency of the interference fringes is equal to the frequency at which the concave portions and convex portions are formed.
In still another embodiment of the present invention, the frequency of the interference fringes is a half of the frequency at which the concave portions and convex portions are formed.
In still another embodiment of the present invention, the optical system is positioned at an opposite side to the recording layer with respect to the guiding portion.
In still another embodiment of the present invention, the optical system is positioned at the same side as the recording layer with respect to the guiding portion.
In still another embodiment of the present invention, the detection section further detects the second light beam reflected by the recording layer so as to output the detection signal; and the optical recording and/or reproduction apparatus further includes a reproduction section for reproducing information recorded in the recording layer based on the detection signal.
In still another embodiment of the present invention, the optical recording and/or reproduction apparatus further includes a recording section for generating a recording current which is used for recording information on the recording layer, wherein the light source emits the first light beam based on the recording current.
According to another aspect of the present invention, there is provided a tracking method for use with an optical recording medium, the optical recording medium including a recording layer and a substrate which has a guiding portion where concave portions and convex portions are periodically formed along a predetermined direction, the method comprising steps of: (a) emitting a first light beam; (b) diffracting a portion of the first light beam so as to output a first diffracted light beam and a second diffracted light beam, and for converging the other portion of the first light beam, thereby outputting a second light beam so as to be converged at a predetermined convergence point; and (c) controlling the optical system, wherein the first and second diffracted light beams interfere with each other to generate interference fringes which extend in a direction substantially perpendicular to the recording layer, the interference fringes include bright portions and dark portions which occur alternately along the predetermined direction, the step (b) includes a step of (d) outputting the first and second diffracted light beams and the second light beam such that the bright portions of the interference fringes form light spots on the guiding portion when the predetermined convergence point of the second light beam is present within the recording layer, the tracking method further includes a step of (e) detecting the interference fringes reflected by the guiding portion so as to output a detection signal, and the step (c) includes a step of (f) controlling the optical system based on the detection signal such that the optical spots follow at least either of the concave portions and the convex portions.
According to still another aspect of the present invention, there is provided an optical recording and/or reproduction apparatus for use with an optical recording medium, the optical recording medium including a recording layer and a substrate which has a guiding portion where concave portions and convex portions are periodically formed along a predetermined direction, the optical recording and/or reproduction apparatus comprising: a light source for emitting a first light beam; an optical system for diffracting a portion of the first light beam so as to output a first diffracted light beam, a second diffracted light beam, a third diffracted light beam, and a fourth diffracted light beam, and for converging the other portion of the first light beam, thereby outputting a second light beam so as to be converged at a predetermined convergence point; and a controlling section for controlling the optical system, wherein the first and second diffracted light beams interfere with each other to generate first interference fringes which extend in a direction substantially perpendicular to the recording layer, the third and fourth diffracted light beams interfere with each other to generate second interference fringes which extend in a direction substantially perpendicular to the recording layer, the first and second interference fringes include bright portions and dark portions which occur alternately along the predetermined direction, the frequency of the first interference fringes and the frequency of the second interference fringes are shifted with respect to each other along the predetermined direction, the optical system outputs the first, second, third, and fourth diffracted light beams and the second light beam such that the bright portions of the first and second interference fringes form light spots on the guiding portion when the predetermined convergence point of the second light beam is present within the recording layer, the optical recording and/or reproduction apparatus further includes a detection section for detecting the first and second interference fringes reflected by the guiding portion so as to output a detection signal, and the control section controls the optical system based on the detection signal such that the optical spots follow at least either of the concave portions and the convex portions.
According to still another aspect of the present invention, there is provided an optical recording medium comprising: a substrate having a guiding portion in which concave portions and convex portions are formed; and m recording layers (m is an integer equal to or greater than 2 (mxe2x89xa72)), where information is recorded in the m recording layers by irradiation of light, wherein the m recording layers includes: a first recording layer, which is a furthest recording layer from a surface of the recording medium on which the light is incident; a mth recording layer, which is a closest recording layer to the surface of the recording medium on which the light is incident; nth and (nxe2x88x921)th recording layers (n is an integer equal to or greater than 2 and equal to or smaller than m (2xe2x89xa6nxe2x89xa6m)), the nth recoding layer being closer to the surface of the recording medium on which the light is incident than the (nxe2x88x921)th recoding layer is, the nth and (nxe2x88x921)th recording layers satisfy the following relationships:
Rnxe2x88x921 greater than Rn and
Anxe2x88x921 greater than An,
where Rn and Rnxe2x88x921 respectively denote the reflectances of the nth and (nxe2x88x921)th recoding layers, and An and Anxe2x88x921 respectively denote the light absorptances of the nth and (nxe2x88x921)th recoding layers.
In one embodiment of the present invention, the first and nth recording layers satisfy the following relationships:
Rn=R1/n
An=A1/n
R1+A1=1
where Rn and R1 respectively denote the reflectances of the nth and first recoding layers, and An and A1 respectively denote the light absorptances of the nth and first recoding layers.
In another embodiment of the present invention, the concave portions and convex portions are periodically formed along the predetermined direction; and the concave portions and convex portions reflect the light.
According to still another aspect of the present invention, there is provided an optical recording and/or reproduction apparatus for use with an optical recording medium, the optical recording medium including a plurality of recording layers and a substrate which has a guiding portion where concave portions and convex portions are periodically formed along a predetermined direction, the optical recording and/or reproduction apparatus comprising: a light source for emitting a first light beam; an optical system for diffracting a portion of the first light beam so as to output a first diffracted light beam and a second diffracted light beam, and for converging the other portion of the first light beam, thereby outputting a second light beam so as to be converged at a predetermined convergence point; and a controlling section for controlling the optical system, wherein the first and second diffracted light beams interfere with each other to generate interference fringes which extend in a direction substantially perpendicular to the plurality of recording layers, the interference fringes include bright portions and dark portions which occur alternately along the predetermined direction, the optical system outputs the first and second diffracted light beams and the second light beam such that the bright portions of the interference fringes form light spots on the guiding portion when the predetermined convergence point of the second light beam is present within one of the plurality of recording layers, the optical recording and/or reproduction apparatus further includes a detection section for detecting the interference fringes reflected by the guiding portion so as to output a detection signal, and the control section controls the optical system based on the detection signal such that the optical spots follow at least either of the concave portions and the convex portions.
According to still another aspect of the present invention, there is provided a tracking method for use with an optical recording medium, the optical recording medium including a plurality of recording layers and a substrate which has a guiding portion where concave portions and convex portions are periodically formed along a predetermined direction, the method comprising steps of: (a) emitting a first light beam; (b) diffracting a portion of the first light beam so as to output a first diffracted light beam and a second diffracted light beam, and for converging the other portion of the first light beam, thereby outputting a second light beam so as to be converged at a predetermined convergence point; and (c) controlling the optical system, wherein the first and second diffracted light beams interfere with each other to generate interference fringes which extend in a direction substantially perpendicular to the plurality of recording layers, the interference fringes include bright portions and dark portions which occur alternately along the predetermined direction, the step (b) includes a step of (d) outputting the first and second diffracted light beams and the second light beam such that the bright portions of the interference fringes form light spots on the guiding portion when the predetermined convergence point of the second light beam is present within one of the plurality of recording layers, the tracking method further includes a step of (e) detecting the interference fringes reflected by the guiding portion so as to output a detection signal, and the step (c) includes a step of (f) controlling the optical system based on the detection signal such that the optical spots follow at least either of the concave portions and the convex portions.
Thus, the invention described herein makes possible the advantages of (1) providing an optical recording and/or reproduction apparatus capable of performing high-accuracy tracking control of a light beam used for recording/reproduction of information on an optical recording medium having a plurality of recording layers or recording planes and a method for performing such a tracking control; and (2) providing an optical recording medium including a plurality of recording layers where the intensities of light beams reflected from the recording layers are constant.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.